A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen or methanol, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.
Electrocatalysts for oxygen reduction typically comprise platinum or platinum alloyed with one or more base metals. The platinum alloy can be used as a high surface area metal black (an unsupported catalyst) or can be deposited onto a conductive carbon substrate (a supported catalyst). EP 450 849 discloses binary and ternary platinum alloy catalysts and their use in fuel cells. EP 557 674 discloses fuel cell alloy catalysts comprising platinum, gold, and two or more metals chosen from nickel, cobalt and manganese. The amount of nickel, cobalt and/or manganese in the catalysts of EP 557 674 is preferably 23 atomic percent of each metal (if two of the metals are present) or 19 atomic percent of the first and second metals and 8 atomic percent of the third metal (if three of the metals are present).
Platinum is an expensive metal, so it is desirable to increase the proportion of lower cost metals in the platinum alloy whilst maintaining or improving catalytic activity. The present inventors have sought to provide improved platinum alloy catalysts that have higher activity than known catalysts or that have similar activity but lower platinum content.